Method and device for handling cellulose pulp

ABSTRACT

The invention concerns a method and a device for the dilution of dewatered cellulose pulp that maintains a consistency of 30% or greater.  
     By shredding of the pulp to a finely divided dry granulate, dilution to a homogeneous consistency in the medium consistency range can take place exclusively through hydrodynamic effects from the addition of dilution fluid. The dilution fluid is added to the granulate at a position at which the granulate is in free fall in a stand pipe  22, 40  and above a level Liq LEV  of diluted pulp in the stand pipe. A number of nozzles are arranged around the periphery of the stand pipe, directed in towards the centre of the stand pipe, obliquely downwards in the direction of fall of the granulate. It is possible through this simplified procedure to avoid completely the conventional dilution screws, and this reduces the investment costs and is operating costs, while at the same time unnecessary mechanical influence of the pulp fibres can be avoided.

The present invention concerns a method according to the preamble of claim 1 and a device according to the preamble of claim 4.

THE PRIOR ART

In association with either one of the bleaching and the delignification of cellulose pulp in bleaching lines, the pulp passes between different treatment steps in which the pulp is subjected to bleaching or the delignifying effect of various treatment chemicals. The treatment typically alternates between alkaline and acidic treatment steps in which typical sequences may be of ECF type (elemental chlorine-free, Cl, in which chlorine dioxide may be used) such as O-D-E-D-E-D, O-D-PO or sequences of TCF-type (totally chlorine-free) such as O-Z-E-P. Other bleaching steps, such as Pa steps and H steps may be used.

The treatment steps may take place either at medium consistency (8-16%) or at high consistency (≧30%), but it is vitally important to wash out after each treatment step degradation products and lignin precipitated during the treatment step and to reduce to a minimum the remaining fraction of fluid, since the latter will otherwise lead to an increased requirement for pH-adjusting chemicals for the subsequent treatment steps and transfer of precipitated lignin and other degradation products, which subsequent step generally takes place at a completely different pH.

Simple vacuum filters with dewatering drums that are partially (typically 20%-40%) immersed in the pulp suspension that is to be dewatered were used in certain older types of washing step after a bleaching step or a delignification step. In these vacuum filters, a bed of pulp forms spontaneously against the outer surface of the drum under the influence of a negative pressure in the interior of the drum, and the pulp bed is drawn up from the pulp suspension by the rotation of the drum and is scraped off with a scraper on the side of the drum that is moving downwards. A consistency higher than 8-14% is generally never achieved for the pulp bed that has been dewatered, due to the limited degree of dewatering that is achieved, and the dewatered pulp that is scraped of can be readily formed to a slurry with a low consistency again in a subsequent collecting trough. The technique used here is a lower degree of dewatering followed by slurry formation with a cleaner filtrate, and this takes place in a series of vacuum filters in order to achieve the required washing effect. For this reason, it is attempted to achieve as high a degree of dewatering as possible before the dewatered pulp is again formed to a slurry with cleaner filtrate before the subsequent treatment stage.

A dominating washing machine on the market for bleaching lines is the conventional dewatering press, or thickening press, in which pulp is applied to at least one outer surface of the dewatering drum and subsequently passes a nip between the drums and acquires a consistency of 30% or greater after the nip. A practical upper limit lies at 35-40%, where a higher degree of dryness cannot be achieved without affecting the strength properties of the fibres negatively. A representative washing press of this type is disclosed in the patent U.S. Pat. No. 6,521,094.

The dewatered mat of cellulose pulp that is fed out from the washing machine's nip must first be shredded due to the high degree of dewatering, which shredding takes place in a shredder screw.

The purpose of the shredder screw has been exclusively to break up the mat of dewatered cellulose pulp and feed it onwards to equipment in which the cellulose pulp is rediluted to a consistency that makes it possible to pump it onwards to the next treatment step.

The redilution thus preferably takes place in association with adjustment of the pH, which after an alkaline wash normally involves the addition of powerful acidifiers, or the addition of acidic return water/filtrate from subsequent process steps, before the subsequent acidic treatment step. These acidic conditions have involved the dilution in general being held well separated from the previous alkaline wash as well as the associated shredder screw, since the alkaline wash can be built from simpler material than that which is normally required for washing machines that resist acidic conditions. Acidic conditions require material that can resist acids, and this is significantly more expensive that other material.

Since the pulp on output from the shredder screw has an extremely high degree of dryness, a consistency of 30% or higher, redilution has generally been carried out in at least one separate dilution screw arranged after the shredder screw where the dilution fluid is added under intensive agitation from the dilution screw in order to achieve a suitable homogeneous consistency that makes pumping to the next step possible. The diluted pulp that is achieved after the dilution screw is fed to a stand pipe in the bottom of which a pump is arranged.

The very high consistency of the pulp after the dewatering step has given rise to the belief that dilution to a homogeneous medium consistency cannot take place unless the dilution takes place under the influence of intensive agitation from the dilution screw. A consistency of 30% or higher of the cellulose pulp is experienced as dry and compact. It can be mentioned for the sake of comparison that medium-consistency pulp is so compact that it is just about possible to walk on this pulp, when it is at the upper part of the consistency range.

The use of a dilution screw at this position, however, increases the requirement for energy, it increases investment costs, it raises the requirement for maintenance and it involves a further mechanical treatment of the pulp which has a negative influence on the strength properties of the pulp.

AIM AND PURPOSE OF THE INVENTION

The present invention is intended to remove the above-mentioned disadvantages and is based on the surprising insight that even if the pulp has been dewatered to give a very high consistency, 30% or more, no mechanical agitation at all is required during the dilution provided that the pulp bed has been shredded to give small granules of a suitable size, and provided that the dilution fluid is added evenly over a flow of the granulated pulp.

It has become apparent, surprisingly, that the granulated pulp, despite its high consistency displays the properties of a sponge, and provided that the dilution fluid is added evenly to a passing flow of granulated pulp, that is not tightly packed, a primary homogenised dilution of the pulp takes place that is fully adequate in order for it to be capable of being pumped onwards to the next bleaching step.

It is sufficient in laboratory experiments with small quantities of well-granulated pulp with a consistency around 30-35% to pour the required amount of fluid to obtain the required consistency into a container with granulated and non-compressed pulp, and the complete mixture has been homogenised to an even consistency after the addition of the fluid totally without mechanical agitation. Observation of the granulated pulp has shown that there lie cavities between the granules, and the fluid rapidly penetrates between the granules through the complete volume of the granules, after which the granules absorb the fluid as sponges.

This primarily homogenised pulp is fully sufficient such that it can be pumped by a subsequent pump, in which a secondary or supplementary homogenisation takes place, and together this ensures that the same degree of homogenisation can be obtained for the subsequent treatment step completely without a dilution screw.

The principal aim of the invention is thus to redilute pulp from a high consistency of 30% or higher without the use of a dilution screw and without intensive mechanical agitation, which reduces losses in the strength of the pulp.

A second aim is to reduce operating costs and maintenance costs for the process equipment in the redilution, since no operation of dilution screw is necessary.

A further aim is to reduce the investment cost of the process equipment. A reduction of both operating costs and investment costs in the process equipment entails a reduction in the cost of manufacturing bleached pulp to an equivalent degree, and this saving is multiplied by the number of washing machines that are used in the bleaching line. No less than six washing machines are included in an O-D-E-D-E-D sequence, and thus the reduction in costs can be significant.

Approximately 50 kW is required solely for the operation of one dilution screw, and the investment cost is approximately SEK 500,000 (depending to a certain extent on requirements on materials, i.e. whether it needs to be acid-resistant or not).

The operating costs per year in an O-D-E-D-E-D bleaching line will be: 6*50 kW*SEK 0.20 (the price for an operator in Sweden)*24 hours*350 days (the number of operating days per year, excluding stoppages)=SEK 500,000 SEK per year; and the investment cost will be: 6*SEK 500,000=SEK 3,000,000.

The capital cost for an interest rate of 5% gives an annual cost of SEK 150,000.

In summary, implementation of the invention involves a total annual saving that approaches SEK 650,000-1,000,000 SEK including maintenance costs and building space (frameworks, etc.) in a bleaching line with a capacity of 1,000 tonnes per day.

Furthermore, availability of the mill increases since six machines can be removed, each of which has an MTBF (mean time between failure).

A further aim is to remove a treatment step between the washing machine and the subsequent pumping, which makes possible a more compact mill and opportunities to place the washing machines at a lower height over the ground in the mill. The washing machines are normally placed at a great height over the ground, and the pulp falls downwards after being washed in the washing machine while it passes through various conditioning steps. If one of these conditioning steps (such as the dilution screw) becomes unnecessary, the building height can be reduced, which in turn gives a saving.

With these aims, the invention is characterised by the characteristics of claim 1 with respect to the method according to the invention, and by the characteristics of claim 4 with respect to the device according to the invention.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a typical treatment step for the pulp in a reactor with a subsequent washing press according to the prior art;

FIG. 2 shows part of the system in FIG. 1 (prior art);

FIG. 3 shows a dilution system according to the invention;

FIG. 4 shows a detail of FIG. 3; and

FIG. 5 shows a view seen from underneath in FIG. 4, seen at the level of the section A-A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a conventional treatment step for cellulose pulp, hereafter denoted “pulp”. The pulp is fed by the pump 1 to a mixer 2 in which necessary treatment chemicals are added. These treatment chemicals can be, for example, oxygen gas, ozone, chlorine dioxide, chlorine, peroxide, pure acid or a suitable alkali for an extraction step, or a mixture of these, and possibly other chemical or additives such as a chelating agent. The pulp is transported after the addition of the necessary chemicals by the mixer 2 to a reactor system 3, here shown in the form of a single-vessel tower 3 of upwards flow. The reactor system can, however, be constituted by simple pipes or by one or several reactors in series, and possibly with the batchwise addition of chemicals between the towers in those cases in which the bleaching processes are compatible and do not require washing between the towers. The treated pulp is fed after treatment in the reactor system 3 to a pulp chute/stand pipe 4, which establishes the buffer volume and static pressure required, to a pump 5 arranged at the bottom of the pulp chute.

The pulp is fed from the pump 5 to a washing machine 7, shown here in the form of a washing press with two drums 7 a, 7 b. The pulp is applied to the drums, here at the 12 o'clock position, and is led by convergent pulp collectors during the addition of washing fluid (not shown in the drawing) to a final dewatering nip between the drums, from where a mat of dewatered pulp is fed upwards to a shredder screw 8.

The drums in FIG. 1 rotate in opposite directions, and the pulp web is dewatered through the outer surface of the drum while the pulp is led approximately 2700 around the perimeter of the drum to the nip.

It is an advantage if the washing press is equivalent to that shown in the patent U.S. Pat. No. 6,521,094. However, any other type of dewatering press or washing press with a drum or drums can be used in which a consistency of 30% or greater is achieved, for example a washing press with a single dewatering drum and a resistance roller, or another types of washing press with two dewatering drums.

The pulp is fed upwards from the nip in the form of a highly compressed mat 20 to a shredder screw 8, whose axis of shredding is arranged essentially parallel with the axes of rotation of the drums. A small oblique mounting of a maximum of 5-100 may, for example, be present if a conical shredder screw is used, where the mat is fed to an inlet slit in the outer casing of a conical shredder screw, where the inlet slit lies parallel with the axes of the drums. The fragmented pulp is led after this shredder screw 8 out from an outlet in the casing of the shredder screw in the flow 21 to a dilution screw 30 that is driven by a motor 31. The dilution screw exposes the pulp to continuous tumbling during the addition of dilution fluid Liq₂, and the pulp is subsequently fed to a stand pipe 40 at its finally conditioned consistency.

The pulp can subsequently be pumped from the stand pipe 40 to the next treatment step of similar type in the bleaching line.

FIG. 2 shows a part of the same process in a different view in which the shredder screw 8 is oriented in the same direction as the dilution screw 30. In this case it is seen more clearly how the highly compressed mat 20 of dewatered pulp is fed into the shredder screw 8. The shredder screw contains a threaded screw 8 a that is driven by a motor 8 c, and which may be equipped with a number of impact pegs 8 b at the end of its outflow, which pegs further beat and break up the shredded pulp. The fragmented flow 21 of pulp particles is fed to fall under its own weight to the subsequent dilution screw 30.

FIG. 3 shows the dilution system according to the invention in a treatment step that is otherwise equivalent to that shown in FIG. 1. The dewatered web of pulp, which has a consistency of 30% or greater, is fed in this case in to the shredder screw 8 in the same way as shown in FIGS. 1 and 2.

However, dilution occurs in the outlet from the shredder screw according to the invention in a significantly simplified manner. It is important that the web/mat 20 of pulp, which has a consistency of 30% or greater, is first fragmented by the shredder screw such that the mat 20 is granulated to a size of granule that has a normal distribution around a dimension in the range 5-40 mm. The granulated pulp is then fed from the outlet of the shredder screw to fall under its own weight in a stand pipe 22 connected to the outer cover of the shedder screw at the end of its outlet. The dilution fluid Liq_(DIL) is subsequently added under pressure into the stand pipe through a number of fluid jets arranged around the periphery of the stand pipe and above a level Liq_(LEV) of diluted cellulose pulp established in the stand pipe. In the embodiment shown in FIG. 3, the upper connection 22 of the stand pipe to the outer casing of the shredder screw has a smaller diameter than the lower part 40′ that lies below. The principle is that the pulp falls under the influence of gravity down through the parts 22, 40′ of the stand pipe, and its lower part 40′ is given a larger diameter in order to be able to establish a suitable buffer volume before the pumping with the pump 41′ at a given level of pulp Liq_(LEV) in the stand pipe 22, 40′.

The amount of dilution fluid Liq_(DIL) added establishes a consistency of the cellulose pulp within the range of medium consistency 8-16%, which is a consistency that allows the pulp to be sent onwards using an MC pump.

The amount of dilution fluid that is required in order to establish the consistency at which the pulp is subsequently pumped is constituted to more than 75-90% of the fluid that is added at the said nozzles arranged above the level that has been established in the stand pipe. A certain amount of chemicals such as acidifiers/alkali or chelating agents may be added at the bottom of the stand pipe 22/40′, but the principal dilution takes place with the dilution fluid above the pulp level established in the stand pipe.

The cellulose pulp at this medium consistency is fed by the pump 41 onwards from the lower end of the stand pipe to subsequent treatment steps for the cellulose pulp.

The dilution of the pulp from high consistency of 30% or greater at the upper part of the stand pipe to a medium consistency of 8-16% before the pumping from the lower part of the stand pipe takes place in this manner exclusively under the influence of the hydrodynamic effect from the addition of the dilution fluid through the said nozzles.

FIG. 3 and FIG. 4 show an embodiment of the manner in which addition of the dilution fluid can be realised. The dilution fluid is added by a pump to a distribution chamber 60 that is arranged concentrically around the stand pipe 22. The pump pressurises the fluid to a suitable level, an excess pressure of approximately 0.1-0.8 bar. Alternatively, high-pressure nozzles can be used, which finely distribute the dilution fluid in the form of fanned plumes of fluid, oriented at a suitable angle relative to the vertical, a suitable angle being 30-90°.

A number of nozzles 62 are arranged at the bottom of the distribution chamber oriented obliquely downwards, in the direction of flow of the granulate, and inwards towards the centre of the flow. The amount of obliqueness in the mounting is appropriately 45±15° relative to the vertical. The oblique orientation downwards is favourable for achieving an ejecting influence on the granulate flow, and for avoiding the risk that the dilution fluid splashes upwards in the stand pipe.

A number of nozzles, at least four, are arranged around the stand pipe 22/40′, preferably with equal distances between them. With a stand pipe 22 having a diameter of 800-1,500 mm, it is appropriate that 10-40 nozzles are arranged around the periphery of the stand pipe. It is appropriate that the distance between neighbouring nozzles is less than 50-300 mm. If high-pressure nozzles with fan-shaped plumes are used, the nozzles can be arranged with a greater distance between neighbouring nozzles. It is important that the dilution fluid is added evenly around the complete circumference of the flow of granulate and at a sufficiently high pressure in order to penetrate to the centre of the granulate flow. The selection of the pressure is a setting carried out on engineering grounds, considering the properties of the actual nozzles used.

The invention can be modified in a number of ways within the scope of the claims. The nozzle 62 for the addition of dilution fluid can, for example, be realised as a simple drilled hole through a thick-walled sheet, of thickness at least 8-10 mm. However, specially designed nozzles are preferred for optimal penetration of the flow of granulate and for even distribution around the complete circumference of the flow, which nozzles preferably generate fan-shaped plumes of fluid. Addition of dilution fluid can also take place at a sufficiently high pressure that the dilution fluid more forms a very finely divided mist in the region that the granulated pulp passes.

Addition of dilution fluid takes place in the preferred embodiment in association with an increase in the area of the stand pipe 22 to a lower part 40′ of the stand pipe having a larger diameter, but it is not necessary that the addition takes place in association with an increase in area.

A small amount may also be added at the outlet end of the shredder screw, with the addition flow directed down towards the stand pipe. But the dilution is to take place principally through the hydrodynamic mixing effect from the addition of the dilution fluid into the flow of granulate. 

1. A method for the removal of dewatered cellulose pulp from a dewatering press, comprising: applying cellulose pulp to an outer surface of two counter-rotating dewatering drums of a dewatering press, the cellulose pulp having an initial consistency in a range of 4-12%; feeding out the cellulose pulp after a final dewatering nip of the dewatering press in a form of a continuous dewatered mat that maintains a consistency of 30% or higher; feeding the dewatered mat perpendicularly to a shredder screw having an axis of shredding that is arranged essentially parallel to axes of rotation of the dewatering drums and the shredder screw having an outlet defined therein at one end of a surrounding outer cover; shredding the dewatered mat in the shredder screw to produce a granulated pulp having a normalized granulate size distribution in a range of 5-40 mm; feeding out the granulated pulp from the outlet of the shredder screw to fall freely in a stand pipe connected to an outlet end of the shredder screw; adding a dilution fluid under pressure into the stand pipe through a number of fluid jets arranged around a periphery of an upper end of the stand pipe and above a surface liquid level (Liq_(LEV)) of diluted granulated pulp established in LOWER END OF the stand pipe, the added dilution fluid establishing a consistency of the diluted granulated pulp in a range of medium consistency 8-16%; more than 75-90%, of the added dilution fluid being added through the fluid jets arranged above the surface liquid level (Liq_(LEV)) established in a lower end of the stand pipe; exclusively diluting the granulated pulp under an influence of hydrodynamic effect from the addition of the dilution fluid through the fluid jet to reduce the consistency of the granulated pulp from 30% or higher at an upper end of the standpipe to a medium consistency of 8-16% at the lower end of the standpipe without using any mechanical agitators in the standpipe; and feeding the diluted granulated pulp onwardly to subsequent treatment steps for the diluted granulated pulp by pumping the diluted granulated pulp from the lower end of the stand pipe.
 2. The method according to claim 1 wherein the addition of dilution fluid from the fluid jets takes place in a form of pressurized fluid jets directed obliquely downwardly in a fall direction of the granulated pulp in the stand pipe
 3. The method according to claim 1 wherein the fluid jets are directed at an angle of 45±15° relative to a vertical direction and a fall direction of the granulated pulp.
 4. A device for the removal of dewatered cellulose pulp from a dewatering press comprising: a first rotatable drum in operative engagement with a second rotatable drum so that a dewatering nip is formed therebetween; a dewatered mat disposed in the dewatering nip; a shredder screw having an axis of shredding parallel to an axis of rotation of the rotatable drums, the shredder screw having an inlet defined therein for receiving the dewatered mat, the dewatered mat extending from the dewatering nip into the inlet of the shredder screw; the shredder screw having means for shredding the dewatered mat into granulated pulp; the shredder screw having an outlet defined therein; a vertical stand pipe having an upper end connected to the shredder screw and in operative engagement with the outlet of the shredder screw, the standpipe having a lower end; the upper end of the vertical standpipe having a pressurized chamber device arranged around a periphery of the standpipe, the pressurized chamber device containing a dilution fluid; a plurality of nozzles extending through a wall of the pressurized chamber device so that an inside of the pressurized chamber device is in fluid communication with the stand pipe; the lower end of the stand pipe having diluted granulated pulp disposed therein, the nozzles being directed towards the granulated pulp to dilute the granulated pulp with the dilution fluid above a level of diluted granulated pulp established in the lower end of the stand pipe; and a pump arranged at the lower end of the stand pipe adjacent to a bottom of the stand pipe for forwarding the diluted granulated pulp to subsequent treatment steps.
 5. The device according to claim 4 wherein at least four nozzles are arranged around the periphery of the stand pipe.
 6. The device according to claim 5 wherein each nozzle is directed g towards a center of the stand pipe and obliquely downwards at an angle 45±15° relative to a vertical direction and a fall direction of granulated pulp.
 7. The device according to claim 6 wherein all nozzles are connected to a common distribution chamber for the dilution fluids which is pressurized with a pressure-raising device means. 